***p <0.001.
2.3. Experimental materials and
procedure
This study compiled a new set of materials to ensure that each image
contains people and is purely emotional due to images from IAPS with or
without people, as well as it specific emotional images affected neural
responses of participants (Ferri et al., 2012; Weinberg & Hajcak,
2010). The formal experiment comprised 60 images (20 sad, 20 happy, and
20 neutral). These images of valence ratings indicated that happy images
were rated as more pleasant (7.65±0.14, ps <0.001) than
neutral (5.09±0.26) and sad images (2.46±0.26), and its also higher for
neutral image compared to sad images (p <0.001).
Additionally, the arousal score was higher for both happy (6.17±0.26)
and sad images (6.17±0.26) compared to neutral images (3.05±0.22,ps <0.001), but no significant difference was found
between the happy and sad images (p =0.98). All images had the
same brightness and contrast, measuring 475
x 355 pixels, and were presented
on a gray background.
The experimental procedure was adapted from the encoding-retrieval
paradigm (Bone et al., 2020), which
required participants to first memorize and then recall or imagine (see
Figure 1). During encoding, each trial started with a 7-word Chinese
title presented for 0.5 seconds, followed by the corresponding image
displayed for 4 seconds (with the title remaining above the image).
Between trials, a fixation cross was presented for 0.5 seconds. During
imagery, each trial began with ”Ready” and remained visible until
participants pressed the spacebar. This step allowed them to adjust
their pace of the task, which is particularly beneficial for depressed
participants (Zhou et al., 2021).
Upon pressing the spacebar, a crosshair appeared for 0.5 seconds,
followed by a title cue displayed for 0.3
seconds. After that, an empty frame
with with dimensions matching the image (475 by 355 pixels) appeared for
3.5 seconds. Participants were instructed to vividly imagine the image
corresponding to the provided title cue within the given frame.
Subsequently, participants rated the subjective vividness and valence of
their mental imagery on a 1–9 scale (max. 2 seconds). During the
inter-trial intervals, a crosshair
was displayed for 2 seconds.
All images randomly divided into 4 blocks, with each block containing 5
sad, happy, and neutral images. Each image was presented three times in
a random order during encoding period and visualized three times in
pseudo-random order according to title cue during imagery period; each
picture was presented or visualized once before any image was repeated.
To ensure participants were imagining the corresponding image,
participants were required to indicate which of four exemplars they
imagined in 7% of the catch trials (Dijkstra et al., 2018).
Participants were asked to complete
practice experiments before the formal experiment.